1. Field of the Invention
The present invention relates to a plasma processing apparatus including a plasma CVD apparatus, a plasma etching apparatus and a plasma ashing apparatus used for manufacturing a semiconductor device, a photosensitive member device for an electrophotography, a line sensor for inputting an image, a flat panel display, an image pickup device, a photovoltaic power device, etc., and to a plasma processing method which can be carried out by using the apparatuses.
2. Related Background Art
Recently, in a process of manufacturing the semiconductor device, etc., the plasma CVD apparatus is industrially practiced. More specifically, since the plasma processing apparatus including the plasma CVD apparatus using a high frequency of 13.56 MHz and a microwave of 2.45 GHz can process regardless of a substrate material, a deposited film material, etc. being a conductive material or an insulating material, the plasma processing apparatus is generally used.
As an example of such a plasma CVD apparatus, a parallel plane plate type apparatus using a high frequency energy will be explained with reference to FIG. 1.
A cathode electrode 3 is arranged in a reaction vessel 1 via a cathode electrode support plate 2.
An earth shield 4 is arranged around the cathode electrode 3 so that a discharge may not be generated between a side portion of the cathode electrode 3 and the reaction vessel 1.
The cathode electrode 3 is connected to a high frequency power 10 via a matching circuit 9 and a high frequency power supply line. A plane plate-shaped substrate 6, on which a film is formed, for carrying out plasma CVD is arranged at a counter electrode 5 arranged parallel to the cathode electrode 3. The substrate 6 is kept at a desired temperature by substrate temperature control means (not shown).
In case of using this apparatus, the plasma CVD is carried out as follows.
After the reaction vessel 1 is evacuated by a vacuum evacuating means 7 until the reaction vessel 1 is highly vacuum, a reactive gas is introduced into the reaction vessel 1 by gas supply means 8, and a predetermined pressure is held.
A high frequency power is supplied to the cathode electrode 3 from the high frequency power 11, and a plasma is generated between the cathode electrode and the counter electrode.
Thereby, the reactive gas is decomposed and excited to form a deposited film on the substrate 6.
In general, an RF energy of 13.56 MHz is used as the high frequency energy. When a discharge frequency is 13.56 MHz, there is such an advantage that a discharge condition can be relatively easily controlled and a film quality of an obtained film is excellent. However, there is such a problem that a usage efficiency of the gas is low and a formation rate of the deposited film is relatively small.
In view of such a problem, a plasma CVD method using the high frequency having about 25 to 150 MHz has been studied.
For example, "Plasma Chemistry and Plasma Processing, Vol. 7, No. 3, p.267 to 273, (1987)" (hereinafter, referred to as "Reference 1") discloses a parallel plane plate type glow discharge decomposition apparatus is used, and a starting gas (silane gas) is decomposed with the high frequency energy having a frequency of 25 MHz to 150 MHz so that an amorphous silicon (hereinafter, a-Si) film is formed.
Concretely, the Reference 1 discloses that while the frequency is changed within a range of 25 MHz to 150 MHz, the a-Si film is formed; that when 70 MHz is used, a film deposition rate becomes maximum, that is, 2.1 nm/sec; that this maximum rate is from about five times to eight times larger than the rate in case of the plasma CVD method using 13.56 MHz; and that the defect density, the optical bandgap and the electroconductivity of the obtained a-Si film are less influenced by an excited frequency.
However, a film formation described in the Reference 1 is a laboratory scale. When a film having a large area is formed, whether or not such an effect can be obtained is not described at all in the Reference 1.
Furthermore, the Reference 1 does not suggest as to whether or not a film is simultaneously formed on a plurality of substrates and a semiconductor device having a large area which can be practical is effectively formed. The Reference 1 merely suggests such a possibility that the use of the high frequency (13.56 MHz to 200 MHz) has an interesting view of a high-speed processing of an a-Si:H thin film device having a large area at a low cost which is required for a thickness of several .mu.m.
The above conventional example is the example of the plasma CVD apparatus which is appropriate for processing a plane plate-shaped substrate. An example of the plasma CVD apparatus which is appropriate for forming a deposited film on a plurality of cylindrical substrates is disclosed in EP154160A (hereinafter, referred to as "Reference 2").
The Reference 2 discloses the plasma CVD apparatus using a microwave energy source having the frequency of 2.45 GHz and the plasma CVD apparatus using a radio frequency energy (RF energy) source.
In the plasma CVD apparatus using the microwave disclosed in the Reference 2, since the microwave energy is used, a plasma density is extremely high when the film is formed. Therefore, the starting gas is rapidly is decomposed, thereby carrying out film deposition at high speed.
Accordingly, there is such a problem that it is very difficult to stably carry out formation of a deposited dense film.
Next, an example of an RF plasma CVD apparatus of a type described in the Reference 2 will be explained with reference to the accompanying drawings.
The plasma CVD apparatus shown in FIGS. 2 and 3 is a plasma CVD apparatus based on the RF plasma CVD apparatus described in the Reference 2.
FIG. 3 shows a cross-sectional view taken in the line 3--3 of FIG. 2. FIGS. 2 and 3 show a reaction vessel 100.
Six substrate holders 105A are concentrically arranged at a predetermined distance in the reaction vessel 100. Numeral 106 denotes a cylindrical substrate for film formation arranged on each substrate holder 105A.
A heater 140 is arranged in an inner portion of each substrate holder 105A so that the cylindrical substrate 106 may be heated from an inner side.
Furthermore, each substrate holder 105A is connected to a shaft 131 coupled to a motor 132 so that the substrate holder 105A may be rotated.
Numeral 105B denotes an auxiliary holding member of the cylindrical substrate 106. Numeral 103 denotes a cathode electrode for introducing a high frequency power located at the center portion of a plasma generating region.
The cathode electrode 103 is connected to a high frequency power source 111 via a matching circuit 109.
Numeral 130 denotes a cathode electrode support member. Numeral 107 denotes an evacuation pipe provided with an evacuation valve. The evacuation pipe is communicated with an evacuation mechanism 135 provided with a vacuum pump. Numeral 108 denotes a starting gas supply system comprising a gas cylinder, a mass flow controller, a valve and the like.
The starting gas supply system 108 is connected to a gas discharge pipe 116 provided with a plurality of gas discharge ports via a gas supply pipe 117. Numeral 133 denotes a seal member.
In case of using this apparatus, the plasma CVD is carried out as follows.
After the reaction vessel 100 is evacuated by the evacuation mechanism 135 until the reaction vessel 100 becomes highly vacuum, a starting gas is introduced into the reaction vessel 100 from the gas supply means 108 via the gas supply pipe 117 and the gas discharge pipe 116, and a predetermined pressure is held.
Next, a high frequency power is supplied to the cathode electrode 103 from the high frequency power source 111 via the matching circuit 109 to generate plasma between the cathode electrode and the cylindrical substrate 106.
Thereby, the starting gas is decomposed and excited by the plasma, and the deposited film is formed on the cylindrical substrate 106.
When the plasma CVD apparatus shown in FIGS. 2 and 3 is used, since a discharge space is surrounded by the cylindrical substrate 106, there is such an advantage that the starting gas can be used at a high utilization efficiency.
By the way, when the deposited film is formed on the entire surface of the cylindrical substrate by using the plasma CVD apparatus shown in FIGS. 2 and 3, it is necessary to rotate the cylindrical substrate. By rotating the cylindrical substrate, a substantial deposition rate is reduced to about 1/3 to 1/5 of the case of using the parallel plane plate type plasma CVD apparatus described above.
That is, since the discharge space is surrounded by the cylindrical substrates, the deposited film is formed at the same deposition rate as the parallel plane plate type plasma CVD apparatus at a position where the cylindrical substrate is just faced to the cathode electrode. However, the deposited film is scarcely formed at the position which is not contacted to the discharge space.
A concrete frequency of the RF energy is not described in the Reference 2.
The plasma CVD apparatus shown in FIGS. 2 and 3 is used, 13.56 MHz which is generally used as the RF energy is used, and SiH.sub.4 is used as the starting gas. Under the pressure condition of several 100 mTorr that a powder such as polysilane, etc. is easily generated, although the deposition rate is high, while the cylindrical substrate is rotated, an amorphous silicon film is deposited on the entire surface of the substrate. In this case, a substantial deposition rate is at most 0.5 nm/s.
When the plasma CVD apparatus shown in FIGS. 2 and 3 is used so that an electrophotographic photosensitive member comprising an amorphous silicon film as a photosensitive layer is manufactured, the thickness of an amorphous silicon photosensitive layer needs about 30 .mu.m. Accordingly, it takes more than sixteen hours to deposit the film at the deposition rate of about 0.5 nm/s described above. Therefore, a productivity is not sufficient.
Furthermore, according to the apparatus shown in FIGS. 2 and 3, when the frequency of the RF energy is 30 MHz or more, a nonuniform plasma is easily formed relative to the axial direction of the cylindrical substrate. There is such a problem that it is extremely difficult to form a uniform deposited film on the cylindrical substrate.